In order to provide a complete understanding of the phenomena regulating the absorption of liquid in a disposable sanitary article, several instruments have been developed by the industry allowing to quantify various parameters of an absorbent system. Perhaps, the most basic of such instruments is the gravimetric absorbency test system (hereinafter "GATS") which is utilised to measure the ultimate absorbent capacity of a porous network. The GATS instrument comprises a horizontal porous plate made of fritted glass in fluid communication with a vertical burette filled with the appropriate test liquid, such as an aqueous solution. The test procedure consists of placing the sample material in contact with the fritted glass plate under constant pressure for allowing the sample material to absorb test liquid under the effect of capillary attraction. The test liquid is supplied at the interface between the fritted glass plate and the sample material under a negative pressure head of one centimeter which is achieved by maintaining the level of test liquid in the burette one centimeter below the top surface of the fritted glass plate. To prevent an undesirable pressure head increase due to the absorption of test liquid from the fritted glass plate an electromechanical control system replenishes the burette at the same rate as test liquid is withdrawn therefrom to maintain the level of test liquid constant.
The absorption of test liquid will cease when the residual capillary attraction of the sample material is counterbalanced by the negative pressure head. The amount of test liquid extracted from the burette is then observed and it is divided by the weight of the sample to obtain the maximal absorbent capacity of the sample material per unit of weight.
The GATS instrument is an extremely useful research tool because it allows to measure with high sensitivity and precision the maximal amount of body exudate that the absorbent system of a sanitary article can retain. Although this parameter is an important design factor during the development of absorbent systems capable to provide adequate protection against leakage and staining of the wearer's undergarment, one must also take in consideration the ability of the absorbent core to desorb the skin-contacting cover layer of the sanitary article which receives the fluid discharge. This is particularly important for absorbent articles such as sanitary napkins which are intended to be worn over a period of time during which several liquid discharges may occur. For such applications, the capability of the absorbent article to rapidly draw liquid away from the point of impact is critical in order to rapidly capture a liquid discharge and also to keep the cover layer of the sanitary article as dry as possible for maintaining the perineal region of the wearer free of moisture.
The GATS instrument does not have the ability of measuring the capillary attraction developed at a surface of a porous network because it functions by allowing a controlled liquid transfer toward the sample material, which necessarily alters the moisture contents at its surface. In contrast, the surface capillary attraction is a parameter which should be observed while no liquid is being delivered to the porous network.
In absence of any direct method for measuring the surface capillary attraction of an absorbent body, the industry has developed some indirect techniques which can which yield more meaningful results than the GATS instrument. All these techniques, however, rely to some degree on a controlled fluid transfer toward the absorbent body, which as discussed above introduces significant inaccuracies in the test results.